Commutator brush assembly



Aug. 9, 1960 Filed July 17, 1958 J. F. BRINSTER EIAL COMMUTATOR ssusuASSEMBLY 2 Sheets-Sheet 1 IN VENTORS.

JOHN F. BRINSTER EVERETT B. GARRETSON AGENT Aug. 9, 1960 J. F. BRINSTERETAL 2,948,795

COMMUTATOR BRUSH ASSEMBLY Filed July 17, 1958 l 2 Sheets-Sheet 2INVENTORS.

JOHN F. BRINSTER EVERETT B. GARRETSON PM KM AGENT United StatesPatentOfificc COMMUTATOR BRUSH ASSEMBLY John F. Brinster and Everett B.Garretson, Princeton, NJ., assignors to General Devices, 'Inc.,Princeton,

NJ., a corporation of New Jersey Filed July 17, 1958, Ser. No. 749,208

7 Claims. (Cl. 200-166) This invention relates to high speed commutatorsor switching devices for electrical signals such as may be used, forexample, in time-division multiplex systems of signal transmission, andit refers more particularly to an improved assembly of brush and brushholder for such devices.

The art of high speed time-division multiplexing, or signal sampling,whereby a plurality of signal channels are sequentially assigned the useof a common trans mission path, such as a wire or radio circuit, makessevere demands on the commutator which connects the sources of thechannel signals, in turn, to the common path. A prime consideration isthat there shall be no significant distortion of the transmitted signalsas a result of the commutation process. This distinguishes commutatorsfor such service, where the informationbearing signals are at a lowlevel, from commutators for. electric generator and motor service wherehigh current carrying capacity is the principal design consideration andminor distortions of the commutated voltage wave can be tolerated.

In a high speed mechanical commutator which comprises a plurality ofspaced contacts over which a resiliently biased brush sweeps, a commonsource of signal or wave distortion is a variation in contact pressureas the brush traverses a contact, with a resultant variation in brushcontact resistance. This, in turn, produces a modulation of thetransmitted signals which may affect the intelligence carried by them orcompletely destroy their utility. In some cases the brush actuallyleaves the surface of a contact, momentarily, while traversing thecontact, thus breaking the channel circuit. Among other causes, variablepressure and contact bounce apparently are related to incipientvibrations of a brush which may be set up normal to the contact surfacein the oscillatory system comprising the brush and the resilient biasingmeans. The present invention, according to one feature thereof, providesa brush assembly for a mechanical commutator of the above type whichincludes improved means for damping or preventing such vibrations,thereby significantly improving signal transmission.

Accordingly, it is one object of the invention to provide improvedcommutator means for the switching of low-level information-bearingsignals.

Another object is to provide a brush assembly for a speed mechanicalcommutator which reduces variations in brush contact resistance.

Another object is to provide a brush assembly of the above type whichhas improved means of simple construction for damping or preventingbrush vibrations.

These and other ojects and advantages of the invention will be moreclearly apparent by reference to the following specification and to theaccompanying drawings in which:

Fig. 1 is a fractional view, to a greatly enlarged scale, of acommutator assembly showing a contact thereof engaged by a cooperativebrush assembly constructed in accordance. with the principles of theinvention;

Fig. 2 is a fractional plan view of the brush-carrying rotor of thecommutator assembly of Fig. 1;

Fig. 3 is an end view of the brush forming a part of the brush assemblyof Fig. 1;

Fig. 4 is a side view, partly in section, of the brush of Fig. 3;

Fig. 5 is a plan view of the stator of the commutator assembly of Fig.1; and

Fig. 6 is a plan view of the rotor of said commutator assembly.

The form of the brush assembly of the invention illustrated anddescribed herein is one that finds utility, more particularly, in acommutator of the flat disc type, but is not limited to such use. In acommon construction for a high speed disc type commutator a plurality ofcontacts are arranged in spaced circular manner on the face of a disc ofinsulating material, usually the stator disc, and are'sw'ept over by atleast one brush mounted on an adjacent rotor disc, or its equivalent.-In addition, a second brush carried by the rotor engages a collectorring on the stator, the two brushes being in electrical connection.Springs individual to the brushes maintain a predetermined contactpressure and allow limited displacement of the brushes to accommodateslight variations in spacing of the brush holders and engaged contactsduring relative displacement thereof. The stator contacts and collectorring usually are made of a precious metal, for example of silver alloy,or have a facing of such a metal and the contact material of the brushis chosen to have a low electrical resistance and a low coeflicient offriction against the surface of such a contact. For multiplexing orsignal sampling service such commutators operate at high switchingrates, a speed of 2000 contacts swept over by a brush in a second notbeing unusual. Precise positioning of the brushes and their dynamicstability, therefore, assume an importance not present in low speedswitching operations if distortion of the signal samples, whose durationis measured in microseconds, is to be avoided.

Referring, now, to the figures and particularly to Fig. 1, there areshown as components of a commutator assembly a stator disc 11 and arotor disc 13, both of insulating material. Stator disc 11 mounts a ringof spaced contacts 15 having shanks pressed or molded into the disc andhaving the heads thereof elevated above the surface of the disc, betterseen in their assembled relationship in Fig. 5. Disc 11 also carriescollector ring 17. Rotor disc 13 mounts brush assemblies 19 and 19', thelatter seen in Fig. 6, comprising brushes engaging contacts 15 andcollector ring 17, respectively. Brush assembly 19 comprises an outershell constituting a brush retainer or brush holder 21, preferably of asilver alloy or a silver plated metal, which has a tubular body portionfitting tightly in a hole in disc 13 and an arcuate end flange held todisc 13 by screws 23. This construction permits the holes in disc 13 inwhich the brush assemblies are inserted to be jig-drilled in positionswhich accurately define the positions of the brushes themselves. Thearcuate shape of the end flange facilitates close spacing of multiplebrushes, where used.

Brush 25 comprises a sleeve portion 27 of ground finished aluminumtubing and a brush head 29 of high conductivity amorphous carbonaceousmaterial such as a metallized graphite preferably a silver-bearinggraphite. The brush head has a cylindrical end portion 30 which ispressed into sleeve 27 and which is of reduced diameter in comparisonwith an adjacent cylindrical portion 31. The latter portion forms anarrow band of substantially less axial extent than sleeve 27, possiblyone tenth to one third the length of the sleeve, with a diameter thatpreferably is of the order of but is not Patented Aug. 9, 1960 one ormore thousandths of an inch greater. The next brush head portion 33 maybe square as shown in Figs. 3 and 4 or rectangular or. even round. Theendportion 35 of brush head. 29 is of square (or rectangular) crosssection, this sectional form being best seen irrFig. 3. The form andextent of the brush contact are a are in part determined by the timingand timing tolerances required. Since the spaced contacts are raisedabove the surface of disc 11, as shown, the brush must always produceshorting action between adjacent contacts. Non-shorting electricaloperation is achieved by providing an electrically isolated contactbetween each pair of live contacts. By way of example of a brush fornon-shorting service the active brush contact surface may be a square0.040 inch on a side for engagement with contacts inch in diameter. Thisdimension will vary with the duty cycle of the switching operation. Thebrush heads are precisely machined with particular reference of therelation of the rectilinear edge of the brush contact surface to thelongitudinal axis of the brush, to the end that precise phasing of theedges of the commutated waves may be attained. The phasing of theleading edges of the waves, as determined by the position of the leadingbrush contact edge, usually is of major importance.

Brush 25, comprising head 29 and sleeve 27, has an easy sliding fit,without appreciable play, in the bore of brush holder 21, where itengages the holder, and is urged away from the holder or retainer towardengagement with a contact 15 by compression spring 37. A limit to thedisplacement of brush in one direction is defined by an inwardlyprojecting end portion of a retainer ring 39 seated in a groove inholder 21 which is adapted to engage, as a stop, the radially extendingsurface or step between brush head portions 31 and 33. Spring 37 has asa stationary abutment a dowel pin 41' fast in holder 19 along a diameterof the bore thereof to accommodate which a slot 43, is provided insleeve 27 (Fig. 4). Pin 41 fits closely in slot 43 and thereby preventstwisting of the brush which otherwise would skew the leading edge of thecontact surface thereof, with a resultant change in phasing of thecommutated wave. Brush assembly 19', cooperative with collector ring 17,may be similar in construction to assembly 19.

'Brush 25 is held to a low mass by employing a minimum amount ofmaterial, for the function performed, in head 29 as well as in sleeve27. This sleeve, in a size compatible with the dimensions of head 29given above, may have an outside diameter of A; inch and a wallthickness of 0.010 inch. An appreciable axial extent is necessary forthe smooth displacement of the brush, without chatter, in the bore ofbrush holder 21. Spring 37 has considerable stiffness to produce asatisfactory brush pressure, which may be of the order of 80 to 100grams. The natural frequency of axial vibrations of the brush is thusraised to a relatively high value, generally above 2000 cycles persecond, which facilitates damping. The slight degree of damping thenrequired to inhibit vibration is introduced principally by the rubbingof the narrow band cylindrical portion Ell of the brush head against theinner surface of the bore of brush holder 21. An axial length of only0.040 for the rubbing surface of brush head portion 31 has been found tobe effective with a brush of the given dimensions, under certainconditions. The same type of material, a metallized graphite, whichprovides low electrical brush resistance has been found to provide asuitable coefficient of friction against the metallic brush retainer fordamping purposes, allowing one member of simple construction to havethese two functions.

As a result of investigation it is believed that axial brush vibrationsresult from a repeated sticking and release of the, brush relative tothe contact over which it ispassing. This stick-slip manner of inducingvibrations 4 is known in other manifestations, for example in the bowingof stringed musical instruments. In high speed commutator practice itusually appears to a noticeable extent only after an initial period ofoperation which results in the deposition of some of the brush materialon the contacts. The sticking possibly therefore may be attributed toseizure of two like surfaces. Again, in the presently disclosed brushassembly construction a portion of the same brush head member whichdeposits material on the contacts serves as an effective means fordamping or preventing vibrations resulting from such deposition.

The description herein of a preferred form of the invention is not to beconstrued as limiting the invention to the described form, equivalentmeans being Within the scope of the invention.

What is claimed is: V

1. In a commutator brush assembly for high speed switching of low levelinformation signals the combine tion of a metallic cylindrical bodybrush retainer having an axial bore, a brush partially mounted withinsaid bore for limiteddisplacement axially thereof comprising a brushhead of high conductivity metallized graphite material having an endportion defining a brush" contact surface for engagement with opposedcommutator contacts and a portion spaced therefrom of cylindricalsection in close sliding frictional engagement with the inner surface ofsaid bore together with a metallic tubu=- lar extension of said headcoaxial with said bore of substantially greater axial extent than saidcylindrical head portion and having a longitudinal slot therein, theoutside diameter of said extension being of the order of but not greaterthan the diameter of said cylindrical head portion, a pin monted on saidbrush retainer and diametrically disposed within said bore forengagement with the slotted portion of said tubular extension'to preventrotation of the brush within said bore; and compression spring meanshaving an abutmenton said' pin and engaging at the free end thereof saidbrush head to urge the brush toward the exterior of the bruslrre tainer.

2. The combination defined in claim 1 wherein the end brush contactsurface of said brush head is bounded by at least one rectilinear edge,to provide the operational leading edge of said surface. 1 i

3. In a commutator brush assembly for high speed switching of low levelinformation signals the combina? tion of a brush retainer having anend-flanged cylindrical; body portion and a bore coaxial therewith, apin fast in said retainer and extending. along a diameter of said bore,a brush at least partially positioned within said bore comprising atubular member longitudinally slotted adjacent one end thereof toreceive said pin and a. brush. head of metallized graphite materialfixedly mounted by said tubular member at the other end thereof andextending axially beyond the member, said brush head having acylindrical portion adjacent to and coaxial with" said tubular member ofat least as great adiameter; as the outside dimeter of said member, saidtubular member and said cylindrical brush head portion both havingsurfaces with a close sliding fit within said here, the" proportions ofsaid surfaces being selected to optimize. damping, said brush headfurther having. an end portion remote from said tubular member ofrectangular section and of reduced area relative to said cylindricalportion thereof providing the brush contact surface; and compressionspring means acting between saidpin'and' said brush to urge the brushaxially away from said brush retainer. Y L

4. The combination defined in claim 3 wherein; said" compression springmeans is positioned within said tubular member.

5. In a commutator brush assembly the combination- Of a metallic brushretainer having a cylindrical bore, a brush at least partiallypositioned within said: bore hav 5 ing a cylindrical body portion and anaxial extension thereof of rectangular section and of reduced arearelative to said body portion, said extension having an end brushcontact surface of silver graphite material and said cylindrical bodyportion having a band thereof of relatively small axial extentcomprising like material, said band being in close rubbing contact withthe inner surface of said bore, means preventing rotation of said brushwithin said bore, resilient means tending to displace said brush axiallyof said bore, and stop means limiting such displacement.

6. The combination defined in claim 5 wherein said end brush contactsurface is square in form.

7. In a commutator brush assembly for high speed operation thecombination of a metallic brush retainer having a cylindrical bore, abrush having an elongated cylindrical portion thereof mounted Withinsaid bore for sliding contact with the surface of the bore and anexternally extending end portion thereof of reduced sectional areahaving at least one axially extending plane surface defining arectilinear edge of the external end surface of said end portion, saidedge being designed to serve as the operational leading edge of thebrush, axially acting spring means urging said brush away from saidretainer, and means preventing rotation of the brush within saidretainer, said cylindrical brush portion comprising a relatively narrowband of metal-bearing graphite material to provide a predeterminedcoefiicient of friction for rubbing against the surface of said bore,and said end portion being of like material to provide a brush contactsurface of high electrical conductivity.

References Cited in the file of this patent UNITED STATES PATENTS728,273 Mix May 19, 1903 1,821,405 Riese Sept. 1, 1931 2,254,277 EhlersSept. 2, 1941 2,813,179 Rugg Nov. 12, 1957 FOREIGN PATENTS 805,904France Sept. 7, 1936

